A gas lift system is a normal artificial lift technique used worldwide for unloading and producing fluid from the perforation intervals below the packer of a subterranean well.
FIG. 1 illustrates a typical gas lift system which utilizes lifting gas supplied from surface via a casing annulus 7 for injecting into the tubing string 6 via gas lift valves installed in the side pocket mandrels 8 or 9 or 10 above the packer 12. The lifting gas is injected into the tubing string 6 as gas bubbles 11. These gas bubbles 11 decrease the hydrostatic pressure of the fluid column exerting on the perforation intervals (15, 16) below the packer 12. Therefore, the hydrocarbon fluids from the said perforation intervals can flow to the wellbore and to the surface.
In general, the lifting efficiency of a typical gas lift well is governed by many parameters. One which mainly affects the lifting efficiency is an injection depth. It is well-known that the deeper the gas injection depth, the better the lifting efficiency and production of the well can be expected.
With respect to FIG. 1, the maximum gas injection depth of a typical gas lift well is limited by the setting depth of the packer 12 above the top perforation interval 15. Thereby, some gas lift wells which have long vertical distance between perforation intervals for example, hundreds of meters of the vertical distance between top perforations 15 and bottom perforations 16, will suffer in low or no production from the deeper perforation intervals due to poor lifting efficiency.
Methods and techniques in prior art are developed for delivering lifting gas for injecting at downhole below the packer, those techniques are different from the present invention in many aspects, for example, as follows:
U.S. Pat. No. 4,708,595 entitled “INTERMITTENT OIL WELL GAS-LIFT APPARATUS” discloses an intermittent oil well gas-lift apparatus using the sidestring tube running from packer to a bottom hole for delivering lifting gas for intermittent injecting into the chamber at the bottom hole for lifting the liquids flowing therein to the surface.
In contrast, the gas-lift apparatus of the present invention does not run the sidestring tube from the packer to bottom hole and does not inject gas into the tubing or chamber at bottom hole for intermittent lifting the liquid to surface.
Another prior art, US patent application publication number 2006/0076140A1 entitled “GAS LIFT APPARATUS AND METHOD FOR PRODUCING A WELL” discloses a gas-lift apparatus using another tubular member running from the dual-port packer to bottom hole for injecting gas into the wellbore.
However, there still are differences between the gas-lift apparatus of the said US application and the present invention, that is, the gas-lift apparatus of the present invention uses a single tubing string and such tubing string is used for both producing the well and injecting gas to the wellbore at the bottom hole.
The concept idea of the present invention is to improve the lifting efficiency of the fluid in the well by allowing continuous injecting lifting gas to the wellbore at maximum possible depth below the packer by the use of single tubing string. It also maintains good well integrity and well serviceability with the standard tools and techniques already existing in the oil and gas industry. The main difference of the present invention among other prior arts is that the present invention uses only one tubing string running from the surface to the bottom hole for delivering lifting gas for injecting to the wellbore below the packer while other prior arts use additional tube for injecting gas to the wellbore or to the tubing below the packer. The use of new-modified tools in the present invention allows to short bypass lifting gas from the casing annuls above the packer to enter the tubing string at below the said packer. This enables the whole completion string to run as a single completion and allows performing wireline intervention in the future for repairing or changing equipment installed in the tubing string below the packer.